Clocked class D amplifiers can be used for supplying power to electrical drives. Such amplifiers are usually actuated by means of pulse-width modulation (PWM) and in most cases provide electric output current via an LC filter for supplying an electrical drive. The advantages of clocked class D amplifiers are primarily their high efficiency and low heat losses.
Measuring the output current, for example, for regulating or controlling an electrical drive can be carried through a shunt resistor or a transformer with a subsequent analog controller. Such circuit concepts, however, have very little flexibility because they have to be adapted to particular measuring ranges. For example, a shunt resistor has to be selected corresponding to the current range to be measured. In addition, the circuit concepts result in high circuit and adjustment complexity.
Instead of an analog controller it is also possible to use an analog-to-digital converter which can measure the current channels of a plurality of amplifiers, for example, via a multiplexer. However, such a concept causes also high circuit complexity, which is disadvantageous in certain fields of application such as, for example, in the aerospace sector, because here, only a small number of qualified components are available.
Exemplary embodiments of the present invention are directed to an amplifier circuit that enables measuring the output current thereof with a circuit complexity as low as possible.
An idea underlying the invention is to measure the switching edges occurring during switching of switching elements of an output stage of an amplifier circuit, in particular of a class D amplifier, and to determine the output current of the amplifier circuit based on these measurements. In order to minimize tolerance influences of the switching elements, which can be implemented through transistors, a capacitive element, in particular a capacitor, can be connected in parallel to at least one of the switching elements. Since the switching edge of a real switching element, such as a transistor, changes its steepness depending on the output stage current or output current (to be measured), the output current can be derived from the measurement of the switching edge. A switching edge can, in particular, be determined by measuring a delay time of a rising or falling voltage edge. For implementing the amplifier circuit according to the invention, no components are required that are critical in particular fields of application such as the aerospace sector, such as analog-to-digital converters and multiplexers.
One embodiment of the invention relates to an amplifier circuit, comprising
a first and a second switching element that are connected in parallel series between a first and a second voltage potential and are actuated in a clocked manner in amplifier mode, wherein a capacitive element is connected in parallel to at least one of the two switching elements,
a measuring circuit for measuring the switching edges occurring during switching of the switching elements, and
a current-determining circuit for determining the output current by means of the measured switching edges.
The at least one capacitive element connected in parallel can be implemented through a separate capacitor that is configured such that switching edges occurring during switching of the switching elements fall or rise almost linearly with a measurable slope. In particular, the at least one capacitive element is selected such that tolerance influences of the switching elements are reduced to such an extent that their influence on measurements is negligibly low.
The measuring circuit can comprise a time-to-digital converter for measuring a time period of a change of the voltage at the midpoint of the series connection of the two switching elements, which said change occurs during switching of at least one of the two switching elements. A time-to-digital converter enables highly accurate time interval measurements, as a result of which also relatively steeply falling voltage edges can result in time measurements usable for the purposes of the present invention.
The measuring circuit can be configured to generate a start signal for the measuring process carried out by the time-to-digital-converter when the voltage at the midpoint of the series connection of the two switching elements exceeds or falls below a first threshold voltage, and to generate a stop signal for the measuring process carried out by the time-to-digital-converter when the voltage at the midpoint of the series connection of the two switching elements exceeds or falls below a second threshold voltage. Through this, the switching edge of the voltage at the midpoint of the series connection of the two switching elements can be measured over a given voltage range so that possible measuring errors caused, for example, by fluctuating supply voltage can be largely eliminated. For example, in the case of a supply voltage of +25 Volt to −25 Volt, a measuring range between +5 Volt and −5 Volt can be defined through the first and second threshold voltages so that fluctuations of the supply voltage have virtually no measurable influence on the measurements.
It is possible that the first and second threshold values are selected to be almost identical so that only one threshold exists for generating the start and stop signals for the measuring process carried out by the time-to-digital converter, which can result in a slightly reduced accuracy, but is simpler to implement for circuit-related reasons. In particular, the two identical threshold voltages can be selected such that they lie approximately in the middle of the supply voltage range of the amplifier circuit.
The measuring circuit can comprise one or a plurality of comparators for detecting when the voltage at the midpoint of the series connection of the two switching elements exceeds or falls below the first and/or the second threshold voltage, and for generating the start and/or stop signal for the time-to-digital converter. Comparators have the advantage that they are also available for critical fields of application of the amplifier circuit, such as the aerospace sector with its particular specifications. For each threshold value, a separate comparator can be provided. If the first and second threshold values are selected to be identical, a single comparator is sufficient resulting in lower circuit-related complexity when implementing the amplifier circuit.
The time-to-digital converter can be implemented through a programmable module, in particular a FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit) or a special time-to-digital converter module.
The current-determining circuit can be configured to determine the output current by means of the measured switching edges, in particular by means of the measured time period of the change of the voltage at the midpoint of the series connection of the two switching elements, which change occurs during switching of at least one of the two switching elements, and by means of the known value of the at least one capacitive element. For example, with the known value of the at least one capacitive element, the time constant, the voltage range defined by the threshold voltages and the measured time period, the current-determining circuit can determine the charging/discharge current of the at least one capacitive element, which corresponds to the output current of the amplifier circuit.
Furthermore, the current-determining circuit can be configured to determine, based on the time period measured for a positive switching edge and based on the time period measured for a negative switching edge, a positive or negative output current, respectively.
The current-determining circuit can also be configured to determine the difference between the time period measured for a positive switching edge and the time period measured for a negative switching edge and, based on the determined difference, to carry out a calibration of the measured switching edges. This enables in particular a precise calibration and results in an almost linear time-to-current curve over a wide range.
The current-determining circuit can further be configured to consider calibration values when determining the output current by means of the measured switching edges, which calibration values have been determined under known operating conditions of the amplifier circuit, in particular in a clocked amplifier mode with a predefined duty cycle such as, for example, 50:50 for generating a known output current.
The first and second voltage potentials can be approximately identical with regard to the absolute value and can have different polarities. In this case, a zero-crossing occurs during a switching process, which said zero-crossing can be reliably determined by a comparator for a rising and also a falling switching edge, and can be used for the measurement.
The first switching element can be a P-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and the second switching element can be an N-channel MOSFET.
The amplifier circuit according to the invention can be part of a class D amplifier circuit, and the first and second switching elements can be actuated with a pulse-width-modulated signal.
Furthermore, the at least one capacitive element can comprise an attenuator for limiting current, wherein the attenuator comprises in particular a parallel connection of a diode and a resistor and is connected between a connection of the capacitive element and a connection of one of the switching elements. The attenuator can avoid damage to the at least one capacitive element caused by high currents which can occur during closing of the switching element thereby effecting short-circuiting the capacitive element.
Further advantages and possible usages of the present invention arise from the following description in connection with the exemplary embodiments illustrated in the drawings.
In the following description, identical, functional identical and functionally connected elements can be designated with identical reference numbers. Absolute values are given hereinafter only as an example and are not to be understood as limiting the invention.